4,6-Diamino-2-thiopyrimidine-based Cobalt Metal Organic Framework (Co-DAT‐MOF): green, efficient, novel and reusable nanocatalyst for synthesis of multicomponent reactions

In this study, Co-DAT‐MOF powder was prepared via the solvothermal method using 4, 6-diamino-2-thiopyrimidine as the organic linker and Co(NO3)2·6H2O. The synthesized catalysts are characterized using XRD, FT-IR, TGA, SEM, BET, NH3-TPD, and ICP-OES techniques. SEM analysis clearly indicated the formation of nanosheet microspheres. NH3-TPD-MS was employed as a means of identifying the various strengths of acid sites and their relative abundance in an attempt to explain the effect of the catalyst surface acid sites. We identified a new acidic feature in Co-DAT‐MOF catalyst, related to the presence of desorption peaks in the NH3-TPD profiles. The activity of Co-DAT‐MOF catalyst for the synthesis of multicomponent reactions correlates with lewis acidity. In addition, Co-DAT‐MOF exhibited excellent performance for the synthesis of pyrroloacridine-1(2H)-one and chromeno [2, 3- d] pyrimidin-8-amines, as well as good reusability and recyclability.

On completion of the reaction (monitored by TLC analysis), the reaction mixture was cooled to 25 °C. At the end of reaction, 20 mL of ethyl acetate was added to the reaction mixture to separate the catalyst using centrifugation. The filtrate was concentrated under vacuum. The crude products were purified by recrystallization by ethyl acetate and n-hexane (1:6).

Results and discussion
The catalytic system has been studied by XRD, FT-IR, TGA, BET, TPD NH 3 , ICP-OES, and SEM techniques. Moreover, Fig. 2 comparatively shows the FT-IR spectra of a free ligand, Co (NO 3 ) 3 .6H 2 O and Co-DAT-MOF.
In the FT-IR spectrum of free ligand (Fig. 2b), the peaks positioned at 1566, 1675, and 2518 cm −1 were related to stertching vibration of C = N, bending vibration of NH, and stertching band of S-H, respectively. The spectrum of the Co-DAT-MOF, the absorption of S-H stretching band was disappeared because of the formation of Co-S bond between 4,6-Diamino-2-thiopyrimidine and Co. In addition, the bending vibration of N-H, for Co-P-MOF structure shifted to the lower wavenumber (1652 cm −1 ) which results demonstrate Co ions can be regarded as the ions modified on the surface of the ligand. The overall morphology of the Co-DAT-MOF (Fig. 3) indicates that microspheres were synthesized using this simple solvothermal method. As shown in the high-magnification SEM images, the microspheres are composed of numerous ultrathin nanosheets.
In order to investigate the crystal structure of Co-DAT-MOF nanocatalyst, we used XRD pattern (Fig. 4). The XRD of the Co-DAT-MOF shows the main peak at 11.78 which corresponds to the standard pattern 46 .
TGA used to shows the weight loss of Co-DAT-MOF under air atmosphere (Fig. 5). The first weight loss of 9 wt% occurs below 300 °C, corresponding to the elimination of bound DMF molecules. It also indicates a loss of one DMF molecule per host lattice. The second weight loss of 21 wt% over the temperature range of 320-550 °C can be ascribed to the decomposition of Co-DAT-MOF and formation of cobalt oxide.   www.nature.com/scientificreports/ The nitrogen adsorption-desorption isotherm of the Co-DAT-MOF shown in Fig. 6 could be categorized as type IV with hysteresis loops in the range of 0.64-0.96 P/P 0 , the existence of abundant pores. The Brunauer-Emmett-Teller (BET) specific surface area was 2.5 m 2 g −1 . The BJH pore size calculations using the adsorption branch of the nitrogen isotherm indicate a micropore peak at about 1.66 nm for Co-DAT-MOF (Fig. 7).
The NH 3 -TPD pattern of the Co-DAT-MOF catalyst is shown in Fig. 8, Table 1 and peaks desorption of NH 3 are observed. The peak observed at (95-300 °C) is attributed to weak acid sites and the peak at a range of temperatures (300-475 °C) to strong acidic sites. In the case of exceeding this temperature (475 °C) during the NH 3 -TPD measurements, the thermal decomposition of samples could occur and the evolved gases could be misinterpreted as ammonia because the employed detector (TCD) did not enable the identify the evolved gases.
Catalytic studies. Applicability of Co-DAT-MOF was investigated for the synthesis of pyrroloacridine-1(2H)-one and chromeno [2, 3-d] pyrimidin-8-amines. In the first part, a direct synthesis of Chromeno [2, 3-d] pyrimidin-8-amines via combination of, 4-Chlorobenzaldehyde, α-naphthol, malononitrile, and ammonium acetate in the presence of Co-DAT-MOF was presented (Fig. 9). Initially, the effect of solvents (PEG, DMF, EtOH, H 2 O) was also investigated and it was observed that the reaction was highly effective with EtOH. Furthermore, the progress of the reaction depended on the amount of Co-DAT-MOF; the reaction was found to complete in the presence of 50 mg of this catalyst. The control experiment confirmed that the reaction did not occur in the presence of Co (NO 3 ) 3 .6H 2 O, and 4, 6-Diamino-2-thiopyrimidine as acatalyst ( Table 2, entry 11-12). The reaction when conducted at room temperature and 60 °C, the yields observed were very low ( Table 2, entries 5 and 6). The ideal temperature for the reaction was found to be 80 °C. Subsequently we performed the synthesis of diverse chromeno [2, 3-d] pyrimidin-8-amines with different substituted aldehyde under optimized reaction condations (Table 3). Both electron-withdrawing and electron-donating substituents on the aldehydes were found to work reasonably well, giving moderate to good yields of the final products. A plausible mechanism for the formation of chromeno [2, 3-d] pyrimidin-8-amines derivatives has been described in Fig. 10.
The catalytic activity of Co-DAT-MOF was evaluated for synthesis pyrroloacridine-1(2H)-one derivatives based on the one-pot three-component reaction of amine, dimedone, and isatin. We performed the reaction by conducting the reaction of aniline, dimedone and isatin as a model to optimize the process conditions in the presence of Co-DAT-MOF (Fig. 11). We also tested the influence of solvents and found out that DMF, and PEG  www.nature.com/scientificreports/   www.nature.com/scientificreports/ afforded the final products (Table 4). However, higher yields were obtained when PEG was used as the solvent. Then, the influence of temperature on the progress of the reaction was evaluated. Lower yields of the desired product was observed while decreasing the reaction temperature. Afterwards the specific amounts of the catalyst (30, 40, 50, 60 mg) were utilized in the process. In addition, the control experiment confirmed that the reaction did not occur in the absence of the catalyst ( Table 4, entry 8). In order to broaden the scope of the developed protocol, a wide range of amines were examined for the synthesis of pyrroloacridine-1(2H)-one ( Table 5). The amines containing the electron-donating as well as electron-withdrawing substituents were compatible under the optimized reaction and provided good to excellent yield of the corresponding pyrroloacridine-1(2H)-one. Moreover, a plausible mechanism on the basis of the previous publications for the synthesis of pyrroloacridine-1(2H)-one derivatives has been shown in Fig. 12.

Heterogeneity studies
Hot filtration. Hot filtration is another technique to know the heterogeneity of a reaction. Hot filtration technique was carried out for the synthesis 4, 4-dimethyl-2-phenyl-4,5-dihydropyrrolo[2,3,4-kl]acridin-1(2H)one. The catalyst was separated from the reaction mixture by a simple filtration when the reaction proceeded past 50% completion. We have observed that no further reaction occurred after the separation of the catalyst which means that the Co catalyst remains on the surface during the reaction.
One of the most important features of the catalyst is the ability to be recycled. To this aim, the reusability of the mesoporous catalyst has been investigated for the synthesis of 4, 4-dimethyl-2-phenyl-4, 5-dihydropyrrolo [2,3,4-kl] acridin-1(2H)-one using the reaction aniline, dimedone, and isatin. Figure 13 displays that the catalyst could be retrieved by simple filtration and recycled at least 4 times without important loss of its high catalytic activity. Also, the amounts of cobalt leaching after recycling of catalyst was analyzed using ICP-OES. Base on such analysis, the amounts of cobalt in fresh and reused catalyst are 0.065 mol.g −1 and 0.061 mol.g −1 respectively, which shows that cobalt leaching from Co-DAT-MOF is very low.
The recovered catalyst was analyzed to prove stability and the recoverability using FT-IR, and XRD techniques. The FT-IR spectrum and XRD pattern of the recovered Co-DAT-MOF indicate that this catalyst can be recycled without any change in its structure (Figs. 4 and 14).
Comparison of the catalyst. The activity of the prepared catalyst for synthesis of chromeno [2, 3-d] pyrimidin-8-amines was compared with previously reported data in the literature. From Table 6, it is clear that Co-DAT-MOF worked remarkably well to give the desired product within 60 min in 98% yield in shorter reaction.

Conclusions
The Co-DAT-MOF was successfully synthesized using a facile solvothermal method and characterized using XRD, FT-IR, TGA, BET, TPD-NH 3 , ICP-EOS, and SEM techniques. The Co-DAT-MOF particles have a microspheres shape, and good thermal stability. To explore the acidic properties of applied Co-DAT-MOF, the NH 3 -TPD technique was employed. The peak observed at (95-300 °C) is attributed to weak acid sites, and the peak at range of temperatures (300-475 °C) to strong acidic sites. The N 2 sorption isotherm shows that Co-DAT-MOF possesses type IV sorption isotherm. The catalyst was found to be highly efficient and could be reused for four catalytic cycles. This study provides a novel strategy to synthesis of pyrroloacridine-1(2H)-one and chromeno [2, 3-d] pyrimidin-8-amines. www.nature.com/scientificreports/ www.nature.com/scientificreports/   www.nature.com/scientificreports/